sysfs: Add crash_notes_size to export percpu note size

For percpu notes, we are exporting only address and not size. So
the userspace tool kexec-tools is putting an upper limit of 1024
and putting the value in p_memsz and p_filesz fields. So the patch
add the new sysfile crash_notes_size to export the exact percpu
note size and let the kexec-tools parse it intead of using 1024.

The idea came from Vivek Goyal. And a later patch will be sent to
kexec-tools to let it parse the size.

Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Acked-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>